1. Field of the Invention
The present invention relates to an optical information recording medium with a recording layer in which information is recorded/reproduced by the irradiation of laser light or the like, and relates to a recording/reproducing apparatus and a recording/reproducing method for the same, and a method for manufacturing an optical information recording medium.
2. Related Background Art
Attention is being given to an optical recording medium as a large-capacity and high-density memory, and a so-called erasable type capable of rewriting currently is being developed. Such erasable type optical information recording media include one in which a thin film whose phase is changed between an amorphous state and a crystal state is formed as a recording layer on a substrate, whereby information is recorded and erased by means of thermal energy by the irradiation of laser light.
As a phase-changing material constituting this recording layer, an alloy film containing Ge, Sb, Te, In and the like as a main component, e.g., a GeSbTe alloy is known. In general, the recording of information is carried out through the formation of a mark by changing the recording film partially to amorphous, whereas the erasing of information is carried out by changing the thus formed amorphous mark to crystalline.
Changing a recording layer to amorphous is carried out by heating the recording layer at a melting point or higher, followed by cooling at a rate of a predetermined value or more. On the other hand, changing the recording layer to crystalline is carried out by heating the recording layer at a temperature in a range from a crystallization point to a melting point, inclusive.
Information recorded on the recording layer is reproduced by utilizing a difference in reflectance between the amorphous mark and the crystal region. To this end, the recording layer generally is configured so that the reflectance in an amorphous state becomes lower than that in a crystal state.
Also, in general, a spiral-shaped or concentric guide groove is formed on a substrate beforehand for the purpose of tracking by laser light. A region between the grooves is called a land, and in a general configuration only one of the groove and the land is allocated to an information track for recording information and the other one is allocated to a guard band for separating adjacent information tracks from each other. This method is adopted for a recordable CD (CD-R) and a minidisk (MD) also.
As a format of recording information onto an optical recording medium, a PWM recording method is available, in which different lengths of marks are formed to have different lengths of spaces (i.e., positions of a front edge and a rear edge of a mark) therebetween so as to carry the information. In this PWM recording method, if pulse conditions such as a strength of a laser pulse and a generation timing of the same are inappropriate when recording, a heat generated at a front portion of the mark might cause a temperature rise at a rear portion of the mark, resulting in a mark shape where the front portion is thin while the rear portion is thick and deformed, or heat generated at the formation of a mark might exert an adverse influence on the formation of an adjacent mark, so that edge positions of marks fluctuate, resulting in a degradation of signal qualities.
The optimum pulse condition significantly depends on the properties of an optical recording medium and a recording/reproducing apparatus. Accordingly, every recording procedure, where an optical recording medium is mounted on a recording/reproducing apparatus and the recording/reproducing apparatus is started, requires a learning operation for determining an optimum pulse condition. This learning operation is to perform test recording while changing a pulse condition and to compare results obtained from the measurement of the qualities of the reproduced signals so as to determine an optimum recording/reproducing condition. According to such a condition, information is recorded on the optical recording medium.
FIGS. 15 and 16 show one example of such a conventional optical recording medium. FIG. 15 is a perspective view showing a conventional optical information recording medium 90, and FIG. 16 is an enlarged plan view showing a track pitch of tracks formed in the conventional optical information recording medium 90.
In FIG. 15, the optical recording medium 90 includes a transparent substrate with a thickness of 1.1 mm, made of polycarbonate, where a central aperture 2 for mounting the same to a recording/reproducing apparatus is formed at its center. On the transparent substrate, a recording layer is formed. On the recording layer, a protective layer with a thickness of 0.1 mm is formed. The recording layer is irradiated with laser light via this protective film, whereby information is recorded/reproduced.
In the transparent substrate, a track 6 is formed so as to track by laser light during recording and reproducing. In the transparent substrate provided in the optical recording medium 90, a lead-in region 3 exclusively used for reproduction, a test recording region 4 and an information recording region 5 are arranged, where the lead-in region 3 is a region for recording identification information for identifying the optical recording medium 90 with an emboss pit or the like, and the test recording region 4 is a region for performing a learning operation for determining an optimum pulse condition.
With reference to FIG. 16, a track in the test recording region 4 and a track in the information recording region 5 have the same width and space. In addition, a track pitch Tp1 of the tracks formed in the test recording region 4 and a track pitch Tp2 of the tracks formed in the information recording region 5 have the same width.
Meanwhile, a tracking error signal for controlling laser light to track the guide grooves during recording and reproducing is obtained generally by a detecting method called a push-pull method.
FIG. 17 is a block diagram showing the configuration of a conventional recording/reproducing apparatus in which tracking control is carried out by the push-pull method. The recording/reproducing apparatus shown in FIG. 17 includes: a spindle motor 10 for rotating the optical information recording medium 90 mounted thereto; a controller 11; a modulator 12 for modulating data to be recorded on the optical information recording medium 90 into a recording signal; a laser driving circuit 13 for driving a semiconductor laser according to the recording signal; an optical head 14 for focusing laser beams from the semiconductor laser provided therein onto the optical information recording medium 90 to record information and for obtaining a reproduction signal from the light reflected from the optical information recording medium 90; an adding amplifier 15 for outputting a sum signal 15S of electric signals output from photoreceivers 21 and 22 of a photodetector 20 provided in the optical head 14, a binarizing circuit 17 for binarizing the sum signal 15S; a differential amplifier 16 for outputting a difference signal 16S between the electric signals output from the photoreceivers 21 and 22; a data demodulation circuit 18 for demodulating data recorded on the tracks formed in the optical information recording medium 90 according to a binarized sum signal 17S and a tracking control circuit 19 for controlling the optical head 14 so that a laser beam appropriately scans the tracks formed in the optical information recording medium 90 according to the difference signal 16S.
FIG. 18 schematically shows a cross-section of a main portion of the optical information recording medium 90 shown in FIG. 15 along a radial direction of the same and a tracking error signal obtained when a laser beam traverses the track 6 formed in the optical information recording medium 90. In order to carry out the tracking control with stability, it is preferable that this tracking error signal has sufficient amplitude and the fluctuation among the tracks is small. The amplitude of the tracking error signal depends on a width and a depth of the track, a track pitch and a reflectance, where a large amplitude can be obtained with increases in the track pitch and the reflectance.
Recently, improvements in the processing capability of various information equipment have led to increases in the amount of information to be handled. Therefore, a recording medium capable of recording/reproducing a larger amount of information has been demanded. As means for handling the larger amount of information, there are a method in which a track pitch is reduced so as to increase the recording density and a method in which a plurality of recording layers is provided where information is recorded/reproduced onto the respective recording layers from a surface on one side.
However, the reduction in the track pitch for increasing the recording density makes the tracking error signal small, and therefore the tracking error signal becomes susceptible to a change in the reflectance due to the recording of a signal. Accordingly, when test recording is conducted under such a laser pulse condition that an amorphous region becomes extremely large during a learning operation for determining an optimum recording condition, the tracking becomes unstable, thus causing a failure in accessing the track. As a result, problems such as it taking a long time to carry out the learning occur, and moreover a region for the test recording is consumed unnecessarily.
In the case where a track pitch is set large enough to carry out the tracking control with stability even when test recording is carried out under an improper recording condition, the track pitch Tp2 in the information recording region, where the recording is carried out under the optimum recording condition only, also is set larger than required, and therefore a problem of a decrease in the recording capacity occurs.
In the case where information is recorded/reproduced with respect to a plurality of recording layers provided in the optical information recording medium from the surface on one side, the following problems occur. A recording layer located farther away from the laser beam irradiation side is irradiated with an attenuated laser beam due to the passage through recording layers located on the laser beam irradiation side, resulting in a decrease in the amount of the light reflected from the layer located farther away from the laser beam irradiation side. Moreover, in order to enable the recording/reproducing with respect to a recording layer located farther away from the laser beam irradiation side, recording layers located on the laser beam irradiation side have to have a sufficient transmittance, which means that a reflectance thereof has to be decreased. As a result, the tracking error signal becomes small, so that the tracking error signal becomes susceptible to a change in the reflectance due to the recording of a signal. Therefore, as in the above case of narrowing the track pitch, during a learning operation for determining an optimum recording condition, the tracking becomes unstable, which might cause a failure in accessing the track. As a result, problems such as it taking a long time to carry out the learning occur, and moreover a region for the test recording is consumed unnecessarily. In the case where a track pitch is set large enough to carry out the tracking control with stability even when test recording is carried out under an improper recording condition, the track pitch Tp2 in the information recording region, where the recording is carried out under the optimum recording condition only, also is set larger than required, and therefore a problem of a decrease in the recording capacity occurs.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a large-capacity optical information recording medium capable of reliable tracking control during test recording, a recording/reproducing apparatus and a recording/reproducing method for the same and a method for manufacturing an optical information recording medium.